Sulfur is an objectionable element which is nearly ubiquitous in fossil fuels, where it occurs both as inorganic (e.g., pyritic) sulfur and as organic sulfur (e.g., a sulfur atom or moiety present in a wide variety of hydrocarbon molecules, including for example, mercaptans, disulfides, sulfones, thiols, thioethers, thiophenes, and other more complex forms). Organic sulfur can account for close to 100% of the total sulfur content of petroleum liquids, such as crude oil and many petroleum distillate fractions. Crude oils can typically range from close to about 5 wt % down to about 0.1 wt % organic sulfur. Those obtained from the Persian Gulf area and from Venezuela (Cerro Negro) can be particularly high in organic sulfur content. Monticello, D. J. and J. J. Kilbane, "Practical Considerations in Biodesulfurization of Petroleum" IGT's 3rd Intl. Symp. on Gas, Oil, Coal, and Env. Biotech., (Dec. 3-5, 1990) New Orleans, La., and Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389.
The presence of sulfur has been correlated with the corrosion of pipeline, pumping, and refining equipment, and with premature breakdown of combustion engines. Sulfur also contaminates or poisons many catalysts which are used in the refining and combustion of fossil fuels. Moreover, the atmospheric emission of sulfur combustion products such as sulfur dioxide leads to the form of acid deposition known as acid rain. Acid rain has lasting deleterious effects on aquatic and forest ecosystems, as well as on agricultural areas located downwind of combustion facilities. Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389. To combat these problems, several methods for desulfurizing fossil fuels, either prior to or immediately after combustion, have been developed.
One technique which is employed for pre-combustion sulfur removal is hydrodesulfurization (HDS). This approach involves reacting the sulfur-containing fossil fuel with hydrogen gas in the presence of a catalyst, commonly a cobalt- or molybdenum-aluminum oxide or a combination thereof, under conditions of elevated temperature and pressure. HDS is more particularly described in Shih, S. S. et al., "Deep Desulfurization of Distillate Components", Abstract No. 264B AIChE Chicago Annual Meeting, presented Nov. 12, 1990, (complete text available upon request from the American Institute of Chemical Engineers; hereinafter Shih et al.), Gary, J. H. and G. E. Handwerk, (1975) Petroleum Refining: Technology and Economics, Marcel Dekker, Inc., N.Y., pp. 114-120, and Speight, J. G., (1981) The Desulfurization of Heavy Oils and Residue, Marcel Dekker, Inc., N.Y., pp. 119-127. HDS is based on the reductive conversion of organic sulfur into hydrogen sulfide (H.sub.2 S), a corrosive gaseous product which is removed from the fossil fuel by stripping. Elevated or persistent levels of hydrogen sulfide are known to inactivate or poison the chemical HDS catalyst, complicating the desulfurization of high-sulfur fossil fuels.
Moreover, the efficacy of HDS treatment for particular types of fossil fuels varies due to the wide chemical diversity of hydrocarbon molecules which can contain sulfur atoms or moieties. Some classes of organic sulfur molecules are labile and can be readily desulfurized by HDS; other classes are refractory and resist desulfurization by HDS treatment. The classes of organic molecules which are often labile to HDS treatment include mercaptans, thioethers, and disulfides. Conversely, the aromatic sulfur-bearing heterocycles (i.e., aromatic molecules bearing one or more sulfur atoms in the aromatic ring itself) are the major class of HDS-refractory organic sulfur-containing molecules. Typically, the HDS-mediated desulfurization of these refractory molecules proceeds only at temperatures and pressures so extreme that valuable hydrocarbons in the fossil fuel can be destroyed in the process. Shih et al.
Recognizing these and other shortcomings of HDS, many investigators have pursued the development of commercially viable techniques of microbial desulfurization (MDS). MDS is generally described as the harnessing of metabolic processes of suitable bacteria to the desulfurization of fossil fuels. Thus, MDS typically involves mild (e.g., physiological) conditions, and does not involve the extremes of temperature and pressure required for HDS. Additionally, the ability of a biological desulfurizing agent to renew or replenish itself is viewed as a potentially significant advantage over physicochemical catalysis.
The discovery that certain species of chemolithotrophic bacteria, most notably Thiobacillus ferrooxidans, obtain the energy required for their metabolic processes from the oxidation of pyritic (inorganic) sulfur into water-soluble sulfate has stimulated the search for an MDS technique for the desulfurization of coal, in which pyritic sulfur can account for more than half of the total sulfur present. Recently, Madgavkar, A. M. (1989) U.S. Pat. No. 4,861,723, has proposed a continuous T. ferrooxidans--based MDS method for desulfurizing coal. However, a commercially viable MDS process for the desulfurization of coal has not yet emerged.
Because of the inherent specificity of biological systems, T. ferooxidans MDS is limited to the desulfurization of fossil fuels in which inorganic sulfur, rather than organic sulfur, predominates. Progress in the development of an MDS technique appropriate for the desulfurization of fossil fuels in which organic sulfur predominates has not been as encouraging. Several species of bacteria have been reported to be capable of catabolizing the breakdown of sulfur-containing hydrocarbon molecules into water-soluble sulfur products. One early report describes a cyclic desulfurization process employing Thiobacillus thiooxidans, Thiophyso volutans, or Thiobacillus thioparus as the microbial agent. Kirshenbaum, I., (1961) U.S. Pat. No. 2,975,103. More recently, Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389, and Hartdegan, F. J. et al., (May 1984) Chem. Eng. Progress 63-67, have reported that such catabolic desulfurization of organic molecules is, for the most part, merely incident to the utilization of the hydrocarbon portion of these molecules as a carbon source, rather than a sulfur-selective or -specific phenomenon. Moreover, catabolic MDS proceeds most readily on the classes of organic sulfur molecules described above as labile to HDS.
Although Monticello and Finnerty report that several species of bacteria have been described as capable of desulfurizing the HDS-refractory aromatic sulfur-bearing heterocycles, in particular Pseudomonas putida and P. alcaligenes, this catabolic pathway is also merely incident to the utilization of the molecules as a carbon source. Consequently, valuable combustible hydrocarbons are lost, and frequently the water-soluble sulfur products generated from the catabolism of sulfur-bearing heterocycles are small organic molecules rather than inorganic sulfur ions. As a result, the authors conclude that the commercial viability of these MDS processes is limited. Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389.
None of the above-described desulfurization technologies provides a viable means for liberating sulfur from refractory organic molecules, such as the sulfur-bearing heterocycles. The interests of those actively engaged in the refining and manufacturing of petroleum fuel products have accordingly become focused on the need to identify such a desulfurization method, in view of the prevalence of these refractory molecules in crude oils derived from such diverse locations as the Middle East (about 40% of the total organic sulfur content present in aromatic sulfur-bearing heterocycles) and West Texas (up to about 70% of the total).